you may have more than one problem tied together.
First, get the battery checked under load. then test for "shorts to frame" on all the power cables. if you have less than 100k ohms on any power cable to the frame, you will need to trace and fix that short.
you may also have a bad scr or diode, or wire connection to the gate lead of an SCR.
does it do it with the drive wheels in the air? have you checked the drive motor brushes?
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.